Genesis of Eden

RESISTANCE to the most important biopesticide now in use could
evolve far faster than farmers had hoped. A new study of diamondback
moths exposed to toxins made by the bacterium Bacillus thuringiensis
suggests that widespread resistance could emerge in just a few
years. The various strains of B. thuringiensis produce about a
hundred Bt toxins. "But only a fraction of these wig kill
any one pest, so pesticide-makers have a limited arsenal,"
says Bruce Tabashnik of the University of Arizona at Tucson, who
led the research. "They know that resistant strains of insects
will evolve, but they have been hoping that a separate genetic
change would be needed to confer resistance to each toxin."
In the diamondback moth, Plutella xylostella, a major pest of
cabbages and other leafy crops, this does not seem to be the case.
"Our work shows a single gene in resistant moths confers
resistance to four different Bt toxins, and this gene appears
to be far more common than anyone believed," says Tabashnik.
While at the University of Hawaii, working with colleagues elsewhere
in the US and in Canada, Tabashnik took moths from two groups-a
field population that had been heavily sprayed with Bt toxins,
and a lab population that had not been exposed to the toxins for
at least 100 generations. In a series of experiments in cross-breeding,
they found that the offspring of survivors of exposure to a single
toxin were resistant to all four toxins. The results are published
in the current issue of Proceedings of the National Academy of
Sciences. Most alarmingly, the researchers found that 21 per cent
of moths in the isolated lab population had the resistance gene.
Geneticists had previously estimated that Bt resistance genes
would be carried by as few as 1 in 10 000 insects. Jonathan Beard

Editorial New Scientist March 8 1997

IT IS always a tragedy when a gift from nature is squandered,
whether it is a river dead from pollution or a forest laid to
waste for timber. Let's hope that the biotechnology industry is
not about to throw away the enormous potential of a simple bacterium,
Bacillus thuringiensis, in its haste to get to market. Properly
used, the bacterium could provide a wonderful way to control insect
pests and help farmers to gain high yields without synthetic pesti-
cides. Misused, however, and pests will grow resistant to it and
its power will be gone for good. Overeager gene clo@ers are already
rushing in where cautious ecologists fear to tread. The latest
research suggests that it's time to slow them down (see p 5).
B. thuringiensis is a soil bactenum with a remarkable ability
to fight insects. If it gets inside the gut of an insect larva,
it grows and multiplies, producing powerful crystalline toxins
which eventually paralyse the gut and kill the insect. No one
knows why the bac- terium does this, but it's proving very useful
because different strains of the bacterium kill different insects
while none of them harm other creatures. In fact, sprays containing
the bacterium make excellent biocontrol agents and have become
popular for their natural, environmentally friendly ability to
control pests. Many organic farmers have come to rely on them.
Not surprisingly, biotechnology companies have also long been
excited about the potential of the bacterium. There has been a
rush to iso- late the genes that produce the various bacterial
toxins and to patent them as fast as possible. Arguments over
who really owns what are already making the lawyers happy: in
the US, at least six companies are involved in legal dis- putes
over Bt technology. If they are placed into the genome of crop
plants, the Bt gene should give them a natural immunity to insect
pests. The toxin genes are currently under test in crops such
as cotton, maize, tobacco, soya bean, potato and tomato and many
appear promising. If all goes well, Bt gene crops should cut the
need for chemical pesticides, protect the environment and increase
crop yields. Unfortunately, all may not go well. In the rush to
get products to market, the Bt gene may start popping up in crops
everywhere. Insect pests will then be under enormous pressure
to develop resistance to the toxins. So after a brief boom, everyone
would have lost.

Biotechnologists have sought security in the numbers of different
Bt genes. There are many varieties of the bacterium and the toxin
it pro- duces. So they argue that even if a pest becomes resistant
to one strain, it should be possible to control it with another.
Unfortunately, that cosy thought has been undermined by researchers
in Tucson who have found that in one pest-the dia- mondback moth-a
single gene change can confer resistance to four different Bt
toxins. That means the scope for rapid spread of resistance might
be much higher than thought. If indiscriminate use of Bt crops
rushes ahead, resistance may spread just as rapidly. Biotechnology
companies do have ideas about how the spread of resistance might
be slowed. Areas where Bt crops are grown could altemate with
areas containing ordinary crops. if a Bt-resistant strain appears,
it should breed with strains which are not resistant from the
surrounding areas. The Bt-resistant strain should therefore not
spread. All well and good. The trouble is we don't know nearly
as much about managing resis- tant pests as we do about cloning
genes. And we don't know how farmers will use the new varieties.
First experiences with Bt crops aren't reas- suring. The agrichemicals
giant Monsanto tried its first commercial planting of Bt cotton
last year and many pests survived. If these tum out to be the
more resistant insects and they go on to breed, then it is easy
to see that farmers may have accidentally begun selecting for
super- resistant strains. Several things must be done. We need
more research on resistance and pest behaviour- and quickly. Without
it, a few years from now and the potential benefits of Bt could
be gone for ever. And also there must be tighter regulation. Currently,
new biocontrol agents are examined much less closely than new
pesticides. That's because we already know the dangers of pesti-
cides. The impact of biocontrol agents needs to be thought through
too. Numerous other biocontrol agents are wait- ing in the wings.
Viruses, nematode worms and fungi are all being studied, as well
as numerous kinds of toxins from plants. They have real potential
to provide bigger crop yields without damaging the environment.
Let's not waste what nature offers by giving the pests a head
start.